Sound recording system



March 7, 1933. A WHITAKER 1,900,679

SOUND RECORDING SYSTEM Filed Oct. 25, 1928 Patented Mar. 7, 1933 UNITED STATES PATENT OFFICE ALFRED WHITAKER, OF WEST DRAYTON, ENGLAND, ASSIGNOR, BY HESNE ASSIGN- EN'I'S, TO RADIO CORPORATION OF AMERICA, OF NEW YORK, N. Y., AOORPOBA- TION OF DELAWARE SOUND RECORDING SYSTEM Application filed October 25, 1928, Serial No. 815,048, and' in Great Britain October 28,1927.

One difliculty which has been met with in ex'isting apparatus is that of accurately controlling the intensity of a beam of light 1n accordance with sounds over the wide range i of frequencies which it is found necessary to record, in order that sounds may be satisfactorily reproduced. m

A device which will operate satisfactorily at low frequencies will frequently not be satisfactory at high frequencies. Furthermore, there are a number of devices such, for

example, as Kerr cells which can be used with good results at relatively high frequencies,

but which are not so satisfactory at lower frequencies. The reason for this is that greaterv amplitudes are required at low frequencies than at high frequencies to give the same impression of intensity to-the ear, and consequently. greater diflerences in intensity of a beam of light have to be used in recording the lower frequencies; and while a Kerr cell operates sufliciently accurately over small amplitudes, it introduces errors due to departure from proportionality between the applied voltage and the intensity of the light at large amplitudes.

According to the present invention two or more beams of light, the intensities of which are controlled in accordance with the sounds to be recorded, are caumd to produce superposed images upon the photo-sensitive surface. In. a preferred form, two beams of lightare employed, one upon each side of the sensitive film. One of the beams may be provided with intensity controlling means adapted to deal mainly with the lower frequency band and the other beam may cover the higher frequencies. One advantage of the invention is that with one beam, it is frequently diflicult or inconvenient to expose the film to sufii- .cient light to expose it fully, whereas with more than one beam, satisfactory exposures can be obtained more readily.

The invention will be described with reference to the accompanying drawing in which Figs. 1 and 2 are diagrammatic illustrations of two recording systems in accordance with the invention and Fig. 3 is a response characteristic.

Referring to Fig. 1, a sensitive film 1 is mounted in known manner so that it can be caused to pass at the desired speed between two optical systems 2 and 3, one arranged upon each side thereof. The optical system 2 comprises an intensity controlling device adapted to work satisfactorily at the lower frequencies, such, for'example, as a mechanical'shutter having a natural frequency in the lower musical range. This shutter is damped in such a manner as to give a fairly flat response characteristic. The opticalsystem 3 upon the sensitive side of the film 1 comprises an intensity controlling device adapted to work efliciently at the higher frequencies. For this purpose, a Kerr or Faraday cell may be used. This device is also damped in such a manner as to give a fairly flat response characteristic. The two two images produced are superposed. Theoptical s stem 3, which is more responsive to the 'gher frequencies, is preferably placed, as shown, upon the sensitive side of the film 1, since more accurate focusing is desirable when dealing with the higher frequencies. The two intensity controlling devices 2 and 3 may both be operated from any suitable amplifying means which may, if desired, be common to both devices and in this case is connected to terminals 6. It will be seen, by reference to Fig. 3,.that, with an arrangement of this type, the overall response characteristic can be made of a very desirable shape. The curve 7 represents the response of the system 2 and the curve 8 represents the response of the system 3. The

overall response characteristic is therefore the line bounding the cross-hatched area.

Fig. 2 illustrates a modification of the recording system shown in Fig. 1 in which only one source of light 4 is used, and light from the source 4 is directed by means of two mirrors 9 and 10 through the optical systems 2 and 3, respectively, on to the film 1. The photographic film 1 is, of course, screened in such a manner that only the light emerging from the systems 2 and 3 can reach the film.

Instead of the mechanical shutter or the Kerr or Faraday cells, other intensity controlling means may of course be employed. For example, the actual intensity of the source of light may be controlled in any known or suitable manner.

The invention is not restricted to the use of dissimilar optical systems. A plurality of similar optical systems may be used if desired, the images produced by the system being superposed upon the film. The photographic sound record 1 may be reproduced in any suitable manner.

I claim:

1. In a sound recording system wherein a beam of light the intensity of which is controlled in accordance with sounds to be recorded is caused to produce an image on a photo-sensitive surface, means for directing a plurality of beams of light on said surface, and means for controlling the intensity of each of said beams in accordance with sounds to be recorded, said controlling means having different frequency response characteristics over the band of frequencies to be recorded.

. 2. In a sound recording system of the type wherein a beam of light the intensity of which is controlled in accordance with sounds to be recorded is caused to produce an image on a photo-sensitive surface, means for directing at least two beams of light on said surface to produce superposed images thereon, and separate means for controlling the intensity of each of .said beams in accordance with the sounds to be recorded, one of said controlling means having maximum response at high frequencies and another at low frequencies within the band to be recorded, whereby a substantially uniform characteristic is obtained over the band.

3. In a sound recording system wherein a beam of light the intensity of which is controlled in accordance with sounds to be recorded is caused to produce an image on a photo-sensitive surface carried on a trans' parent carrier, means for directing a beam of light on each side of said transparent carrier, and means for separately controlling the intensity of each of said beams in accordance with the sounds to be recorded, one of said controlling means on one side of said carrier having maximum response at high frequencies within the band to be recorded and another of said controlling means on the opposite side of said carrier having maximum response at low frequencies within the band to be recorded.

4. In a sound recording system wherein a beam of light the intensity of which is controlled in accordance with sounds to be recorded is caused to produce an image on a photo-sensitive surface carried by a transparent carrier, means for directing a beam of light on each side of said carrier, and separate means for controlling the intensity of each of said beams in accordance with sounds to be recorded, said controlling means on one side of said carrier having a substantially uniform response characteristic throughout one portion of the band of frequencies to be recorded and another of said controlling means on the opposite side of said carrier having a substantially uniform response characteristic throughout another portion of the band to be recorded, whereby a substantially uniform characteristic throughout the entire band is obtained.

5. In a sound recording system wherein a beam of light the intensity of which is controlled in accordance with sounds to be re corded is caused to produce an image on a photo-sensitive surface carried by a transparent carrier, means for directing a beam of light on each side of said carrier, and separate means for controlling the intensity of each of said beams in accordance with sounds to be recorded, said controlling means on the sensitive side of said carrier having a substantially uniform response characteristic throughout the upper portionlof the band of frequencies to be recorded and the controlling means on the opposite side of said carrier having a substantially uniform response characteristic throughout the lower portion of the band of frequencies to be recorded.

6. The method of recording sounds photographically by means of a device controlling a beam of light in accordance with sounds to be recorded which consists in controlling a plurality of beams of light by a plurality of devices the individual response characteristics of which compensate each other and focusing said beams on the film to produce superposed images.

7 The method of recording sounds photographically by means of a device controlling a beam of light in accordance with sounds to be recorded which consists in controlling separate beams of light by devices one of which has maximum response at high frequencies and the other of which has maximum response at low frequencies and focusing said beams of light on the film to produce superposed images.

8. The method of recording sounds photographically on a transparent film by means of a device controlling a beam of light in accordance with sounds to be recorded which consists in controlling a plurality of beams of light by a plurality of devices the individual response characteristics of which compensate each other, focusing at least one of said beams on said film through one side thereof, and focusing another of said beams on the other side of said film to produce superposed images.

9. The method of recording sounds photographically on a transparent film by means of a device controlling a beam of light in accordance with sounds to be recorded which consists in controlling a plurality of beams of light by a plurality of devices the individual response characteristics of which compensate each other, focusing at least one of said beams on said film through one side thereof, and simultaneously focusing another of said beams on the other side of said film to produce superposed images.

In testimony whereof I have signed my name to this specification.

ALFRED WHITAKER. 

